1. Field of the Invention
The present invention relates to a differential drive mechanism for automotive vehicles, and more particularly to a differential drive mechanism of the type which includes clutch means arranged for transmitting drive torque from an input shaft to a set of road wheels when a speed difference is established between the input shaft and the road wheels.
2. Description of the Prior Art
In operation of a conventional differential gear unit arranged for transmitting drive torque differentially to a set of road wheels, one of the road wheels may not be applied with the drive torque in the event that the other road wheel tends to spin. To eliminate such a drawback, a limited-slip differential in the form of a viscous fluid coupling is assembled within the differential gear unit to limit or control the differential action for positive drive of the road wheels. However, this results in a complicated construction of the differential gear unit.
In U.S. Pat. No. 4,721,010 issued on Jan. 26, 1988, there has been proposed a vehicle drive transmission which comprises first and second drive axle assemblies having respective drive input members and respective road wheels, drive means arranged for delivering drive torque simultaneously to each of the drive input members and with a fixed speed relation therebetween, differential gear means arranged in one of the drive axle assemblies for transmitting drive torque differentially and positively between the associated drive input member and the associated road wheels, and slip coupling means arranged in the other drive axle assembly for transmitting drive torque between the associated drive input member and either associated road wheel when a speed difference exists therebetween. The slip coupling means includes a housing defining an enclosure of generally hollow cylindrical configuration, and three sets of annular plates in the housing, one set of the annular plates being positively engaged with the housing for rotation therewith, the other two sets of annular plates being in positive driving connection respectively with the associated road wheels and being freely interleaved with the annular plates of the one set, and all of the annular plates being sealed within the enclosure in contact with a viscous fluid filled therein.
If in the slip coupling means there is a difference in torque transmission characteristic between the two sets of annular plates, drive torque applied to the associated road wheels will differ. This results in deterioration of the straight-travel stability of the vehicle. During assembly of the slip coupling means, there will inadvertently occur a difference in axial space between the one set of annular plates and the other two sets of annular plates. It is, therefore, very difficult to match the torque transmission characteristics of the two sets of annular plates. In a condition where frictional contact of the road wheels with a road surface frequently changes, there will occur changes of traction of the road wheels. This also results in deterioration of the straight-travel stability of the vehicle. Even if the annular plates are assembled in position without causing any undesired difference in axial space therebetween, there will occur a difference in rise of temperature between the two sets of annular plates. This will cause a difference in torque transmission to the road wheels.